Participants undergo microperfusion. A needle is placed in an arm vein for collecting blood samples before and at the end of the experiment for measuring thyroid hormones and other substances in the blood. The skin over the fat pad next to the umbilicus (navel) is numbed and two microperfusion probes are inserted. A salt water solution is infused into the fat pad for one hour and the substances in the fluid are measured as the fluid leaves the fat pad. After 1 hour isoproterenol, a drug that aids in energy production by fat, is added to help determine the level of fat metabolism. After an additional hour small amounts of thyroid hormone are added to the fluid and samples are collected as they leave the body. Two catheters are inserted during each experiment. Small volumes of sterile fluid are circulated continuously in the tubing and samples are collected at regular intervals.